This invention relates primarily to turbomachinery blades and, more particularly, to a tip cap configuration for a turbomachinery blade and a method of installing such a configuration.
A turbomachinery blade is comprised of a retaining or shank section, a platform section and an airfoil section. Since it is desirable that a turbomachine be operated at very high speeds and temperatures, the airfoil section of the blade is preferably formed of relatively thin, convex and concave side walls defining an internal cavity which has provided therein means for cooling the blade side walls. In an air-cooled blade, the cooling air is commonly brought in through passageways in the shank portion of the blade to the airfoil cavity which is divided by a plurality of ribs into heat transfer chambers and serpentine passageways. The cooling air within the cavity acts to cool the blade side walls by a combination of impingement, convection and film cooling, and is then normally discharged out through film holes and the trailing edge holes.
Because of the critical aerodynamic shape of the blade, and the required intricate structure in the cavity portion thereof, the only practical method of fabrication has been by way of the casting process, wherein precision casting techniques are used to fabricate the turbine blade airfoil and shank section by way of the relatively old "lost wax" technique. When such an investment casting is used for the turbine blade, it is desirable to use a ceramic or glass core to form the internal chambers and passageways within the airfoil cavity. This is normally facilitated by leaving open the tip end of the airfoil and allowing the ceramic core to extend outwardly therefrom for purposes of handling and positioning for location during the fabrication process. It then subsequently becomes necessary to seal the tip end of the blade in order to prevent cooling fluid from being wastefully discharged into the gas stream. This open tip end of the blade is generally sealed by means of a tip cap which is joined to the blade in either a permanent manner, or in a semipermanent manner which allows removal and replacement thereof. Attachment of the tip cap can be accomplished in various ways, a number of which are discussed in U.S. Pat. application Ser. No. 530,720, filed Dec. 9, 1974, and assigned to the assignee of the present invention.
As is well known in the art, the satisfactory attachment of the tip cap to the open tip end of the blade is not a simple task. Not only is the tip cap exposed to very high temperatures, but, more importantly, the high speed rotation of the blade tends to expose the tip cap to tremendously high centrifugal forces which tend to separate the tip cap from the blade. Accordingly, in most attachment schemes, there is involved some form of crimping process wherein a portion of each of the blade concave and convex side walls is crimped inwardly to capture the tip cap thereunder. The difficulty with the use of such a process is two-fold. First of all, the performance characteristics of a turbomachine blade are such that if the aerodynamic shape of the blade is altered to any significant degree, the efficiency thereof will be greatly reduced. Therefore, if the airfoil tip ends are bent inwardly so that they do not form a substantially straight-line extension of the adjoining portion of the blade, the performance characteristics will be substantially affected.
The second problem with the crimping process, is that when the side wall material is crimped, there tends to be a longitudinal deformation of the material, since neither the convex nor the concave sides can otherwise accommodate such a crimping process. That is, when a portion of the convex side wall is crimped, there tends to be an excess of material, and a portion thereof may buckle or crack causing the part to be scrapped. However, when a portion of the concave side is crimped toward the airfoil open end, there is a deficiency of material, and a stress condition is created in the blade side wall which tends to stretch the blade material beyond its capability, cause cracking and the necessary scrapping of an otherwise good turbine blade.
It is therefore an object of this invention to provide a turbine blade having superior aerodynamic characteristics.
Another object of this invention is the provision for installing a tip cap on a hollow turbomachinery blade without the necessity of crimping certain portions of the blade.
Still another object of this invention is the provision for an economical and effective method of installing the tip cap on a hollow turbomachinery blade.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.